High-speed measurement of ICEO in transistors and opto-isolators

Abstract

In testing transistor devices, measurements of the collector-emitter leakage current of the devices with the base open (i.e., I CEO ) generally require long measuring intervals. This is due to the necessity of charging the Miller Capacitance of the device under test prior to taking the measurement. While the Miller Capacitance is charging, a small base current flows which makes an accurate reading of the I CEO very difficult. To shorten these measurement times, a voltage is applied to the base of the device to increase the charging rate of the Miller Capacitance. After a predetermined time, the voltage is disconnected thereby opening the base of the transistor device. The I CEO reading is then taken at the emitter of the device after it has stabilized to an approximate level of I CEO . As an alternative to the application of a voltage to the base, a feedback circuit is connected between the emitter and the base of the test device to provide an automatically adjusted driving current to the base. This sets the emitter current at a desired level and rapidly charges the Miller Capacitance. If the desired emitter current is set at the level of maximum allowable I CEO , a go-no go reading of the device can be very rapidly made after the base is open by determining whether the I CEO is increasing or decreasing from the present level.

What is claimed is: 
     
       1. An apparatus for measuring the collector-emitter leakage current of a transistor device when the base is open, wherein the device has a Miller Capacitance equal to the collector-base junction capacitance multiplied by the current gain of the transistor device, comprising: means for applying a D.C. voltage between the collector and the emitter of the transistor device to flow a current through the device to charge the Miller Capacitance;   means responsive to a control signal for applying a D.C. voltage to the base of the transistor device to induce a current to flow through the device to increase the charging rate of the Miller Capacitance;   means responsive to the control signal for disconnecting the D.C. voltage to open the base after it has been applied for a predetermined time; and   means coupled to the emitter of the transistor device for measuring the collector-emitter leakage current of the transistor device after a predetermined time sufficient to allow the collector-emitter leakage current to stabilize to an approximate level after the disconnecting means has opened the base.   
     
     
       2. An apparatus for measuring the collector-emitter leakage current of a transistor device when the base is open wherein the transistor device has a Miller Capacitance equal to the collector-base junction capacitance multiplied by the current gain of the transistor device, comprising: a feedback circuit coupled between the emitter and the base of the transistor device to supply a driving current from the emitter to the base to charge the Miller Capacitance of the device;   means for disconnecting the feedback circuit to open the base and to discontinue the supply of the driving current to the base after a predetermined time sufficient to charge the Miller Capacitance; and   means coupled to the emitter for measuring the collector-emitter leakage current of the transistor device at a predetermined time after the disconnecting means has opened the base.   
     
     
       3. The apparatus of claim 2, wherein the feedback circuit further comprises: monitoring means coupled to the emitter to monitor the emitter courrent; and   means, coupled between the output of the monitoring means and the base of the transistor device, for comparing the emitter current level with a reference current level and automatically adjusting the driving current to the base of the transistor device until the emitter current level equals the reference current level, the predetermined time for disconnecting the feedback circuit being set to allow sufficient time for the equalizing of the emitter current level and the reference current level.   
     
     
       4. The apparatus of claim 3, wherein the reference current level is set at the level of the maximum collector-emitter leakage current allowable for the transistor device, and the measuring time is set at a predetermined time sufficient to indicate whether the collector-emitter leakage current has increased or decreased from said maximum allowable level after the disconnecting means has opened the base of the transistor device. 
     
     
       5. The apparatus of claim 3, wherein the monitoring means comprises a first differential amplifier which converts the emitter current to a first voltage representative of the emitter current level, the reference current level comprises a voltage source preset to represent a desired current level, and the comparator comprises a second differential amplifier coupled to the first voltage and the reference voltage source. 
     
     
       6. The apparatus of claim 5, wherein the measuring means comprises a time-controlled, voltage-measuring circuit coupled to the output of the first differential amplifier to read the first voltage representative of the emitter current level, said voltage-measuring circuit being scaled to provide a voltage output reading which is indicative of the collector-emitter leakage current. 
     
     
       7. The apparatus of claim 3, wherein the disconnecting means comprises a time-controlled switch coupled between the output of the comparator and the base of the transistor device. 
     
     
       8. The apparatus of claim 7 further including a voltage clamping means coupled from a point between the output of the comparator means and the time-controlled switch to ground to prevent the voltage of the comparator means from rising when the time-controlled switch opens the base of the transistor. 
     
     
       9. The apparatus of claim 8, wherein the voltage clamping means comprises one or more diodes, and means for selectively shunting the diodes to adjust the degree of voltage clamping in accordance with the parameters of the transistor device. 
     
     
       10. The apparatus of claim 2, wherein the transistor device is a Darlington transistor amplifier having two or more transistors. 
     
     
       11. The apparatus of claim 2, wherein the transistor device is an opto-isolator comprising: a light-emitting diode coupled to the output of the feedback circuit; and   a transistor amplifier, having one or more transistors, the input base of said transistor amplifier being driven by the light output of the light-emitting diode.   
     
     
       12. A method for measuring the collector-emitter leakage current of a transistor device when the base is open, wherein the transistor device has a Miller Capacitance equal to the collector-base junction capacitance multiplied by the current gain of the transistor device, comprising: applying a D.C. voltage between the collector and emitter of the transistor device to charge the Miller Capacitance;   applying a D.C. voltage to the base of the transistor device in response to a control signal to induce a current to flow through the transistor device to increase the charging rate of the Miller Capacitance;   discontinuing the voltage to the base in response to the control signal after a predetermined time; and   measuring the collector-emitter leakage current of the transistor device at the emitter after a predetermined time sufficient to allow the collector-emitter leakage current to to stabilize to an approximate level after the base has been opened.   
     
     
       13. A method for measuring the collector-emitter leakage current of a transistor device when the base is open, wherein the transistor device has a Miller Capacitance equal to the collector-base junction capacitance multiplied by the current gain of the transistor device, comprising: applying a D.C. voltage between the collector and emitter of the transistor device;   charging the Miller Capacitance of the transistor device with a feedback driving current from the emitter to the base of the transistor device;   discontinuing the feedback driving current to the base after a predetermined time sufficient to charge the Miller Capacitance; and   measuring the collector-emitter leakage current of the transistor device at the emitter at a predetermined time after the base has been opened.   
     
     
       14. The method of claim 13 further comprising: comparing the emitter current level with a reference current level; and   adjusting the feedback driving current to the base of the transistor device until the emitter current level equals the reference current level.   
     
     
       15. The method of claim 14 further comprising: setting the reference current level to the maximum collector-emitter leakage current allowable for the transistor device so that the feedback driving current will be adjusted until the emitter current of the transistor device equals the maximum collector-emitter leakage current allowable; and   setting the predetermined time for measuring the collector-emitter leakage current after the base has been opened at a time sufficient to determine whether the leakage current has increased or decreased from said maximum allowable leakage current.   
     
     
       16. The method of claim 13 further comprising: converting the emitter current to a first voltage representative of the emitter current level;   comparing the first voltage to a reference voltage representative of a desired emitter current level; and   adjusting the feedback driving current to the base of the transistor device until the first voltage equals the reference voltage.   
     
     
       17. The method of claim 13, wherein the transistor device is an opto-isolator comprising a light-emitting diode and a transistor amplifier having one or more transistors, the method further comprising: driving the light-emitting diode with the feedback driving current so that the diode will emit light; and   driving the base of the transistor amplifier with the light emitted from the light-emitting diode.